1. Field of the Invention
The present invention relates to an antireflection coating for infrared light which can be formed on silicon or germanium substrates.
2. Description of Related Art
Infrared light in a specific wavelength range of 3 to 5 .mu.m, which is a wavelength range that is rarely absorbed by the atmosphere, is used for various devices such as an infrared camera, an infrared detector, and an infrared light measuring device. The kinds of materials which can transmit the above-mentioned infrared light is considerably less than those materials which can transmit visible light. Therefore, an optical element, such as a lens, which is formed from a silicon or germanium substrate, is generally used for the above-mentioned devices. However, silicon and germanium have high refractive indices, so that problems can occur in that the resulting reflectance on a surface of the substrate is high (silicon: approximately 30%, germanium: approximately 36%), and the corresponding transmittance therethrough is low. Accordingly, for achieving a higher transmittance and for preventing any ghost and a flare from occurring in a lens system, it is indispensable to form an antireflection coating on the surface.
As such, an antireflection coating in which a layer of zinc sulfide (ZnS) is contiguously formed on a substrate is known (for example, Japanese Laid-Open Patent Application No. Sho. 64-15703, Japanese Laid-Open Patent Application No. Hei. 2-135401, Japanese Examined Patent Publication No. Hei 2-11121, and Japanese Examined Patent Publication No. Hei. 2-13761).
However, zinc sulfide (ZnS) has a relatively bad adhesion to the substrate, so that the layer of zinc sulfide (ZnS) can easily be removed. Thus, there is a problem in that conventional antireflection coatings will be lacking in endurance and can be stripped from the substrate.